In recent years, there has been remarkable progress in information technology, and development of various types of information recording medium for storing information such as magnetic disks, optical magnetic disks and optical disks has been carried out with vigor.
Of these types of information recording medium, in the case for example of a magnetic disk, a magnetic film is formed on at least one surface of a donut-shaped magnetic disk substrate, and recording and playback of information are carried out by a magnetic head sliding over a data zone thus formed on the magnetic disk substrate.
As a method of manufacturing such a magnetic disk substrate, a method has been proposed in which a thin film of a magnetic material is formed directly on at least one surface of a sheet-shaped starting material glass plate that has been manufactured by a float process or the like, without polishing the starting material glass plate (see, for example, Japanese Laid-open Utility Model Publication (Kokai) No. 60-159531).
However, with this manufacturing method, it is difficult to manufacture a magnetic disk having a good planarity sufficient to cope with increases in data zone recording density that have occurred in recent years. At present it is thus still common to polish the starting material glass plate when manufacturing a magnetic disk substrate.
FIG. 1 is a flowchart showing a conventional method of manufacturing such a magnetic disk substrate.
In this conventional method, a starting material glass plate 101 is cut into a donut shape in a disk processing step 102, then the inner and outer peripheral surfaces of the starting material glass plate 101 are processed to predetermined dimensions in an edge processing step 103, then the main surfaces of the starting material glass plate 101 are polished in a surface polishing step 104, then the substrate is strengthened if necessary in a chemical strengthening treatment step 105, and then a finishing washing step 106 is carried out, thus completing the manufacture of the magnetic disk substrate 107.
In the surface polishing step 104, the polishing of the surfaces of the starting material glass plate 101 is carried out in three stages, i.e. rough polishing 104a, pre-polishing 104b, and precision polishing 104c. 
The surfaces of the magnetic disk substrate 107 manufactured from the sheet-shaped starting material glass plate 101 have an undulating shape on a microscopic scale, in which a plurality of types of surface waviness classified by wavelength band are superimposed on one another, for example, long-wavelength waviness 108, medium-wavelength waviness 109 and short-wavelength waviness 110, as shown in FIG. 2. A magnetic head 111 flies over the magnetic disk substrate 107 having this surface waviness.
However, as the data zone recording density has been increased in recent years, the surface waviness properties have come to exert a large influence on the electromagnetic conversion properties. If the ability of the magnetic head to follow the surface waviness is poor, then there will be a risk of malfunction during recording and/or playback, and hence a very high degree of planarity has come to be demanded of magnetic disk substrates.
The surface polishing step 104 is thus conventionally divided into three stages as described above. First, rough polishing 104a using abrasive grains having a relatively large mean diameter is carried out. As a result, the thickness of the starting material glass plate 101 is adjusted to a predetermined value, and moreover surface waviness, in particular long-wavelength waviness, is reduced, thus correcting the degree of flatness of the starting material glass plate 101. Then, in the pre-polishing 104b and precision polishing 104c, minute flaws formed on the surface of the starting material glass plate 101 and surface waviness of relatively short wavelength (medium- and short-wavelength waviness) are removed.
In recent years, to cope with increases in data zone recording density, there has been vigorous development of technology to reduce the size of the magnetic head 111, set the flying height of this small magnetic head 111 to be low, and make the magnetic head 111 fly stably over the magnetic disk substrate 107 at this low flying height. Nowadays, the length of magnetic heads has been reduced from about 2 mm to 1 mm or less.
As shown in FIG. 3, the long-wavelength waviness 108 is a relatively gently sloping waviness, and hence it is possible for the magnetic head 111 to fly over the magnetic disk substrate 107 while following the long-wavelength waviness 108 and thus maintaining a constant minute gap t between the magnetic head 111 and the magnetic disk substrate 107.
In contrast, as shown in FIG. 4, the medium-wavelength waviness 109 and the short-wavelength waviness 110 have steep sloping parts 112, and hence the magnetic head 111 cannot fly over the magnetic disk substrate 107 while maintaining a constant minute gap t between the magnetic head 111 and the magnetic disk substrate 107 as is possible with the long-wavelength waviness 108, i.e. the magnetic head 111 cannot follow the medium-wavelength waviness 109 and the short-wavelength waviness 110. If medium-wavelength waviness 109 and short-wavelength waviness 110 are present on the substrate surface, then this may thus cause malfunction during recording and/or playback. To obtain a magnetic disk substrate of a desired high quality sufficient for coping with increased data zone recording density, it is thus necessary to carry out surface polishing so that medium-wavelength waviness 109 and short-wavelength waviness 110 are removed.
However, in the conventional manufacturing method described above, although in the rough polishing 104a it is possible to correct the degree of flatness, a lack of which is caused by long-wavelength waviness 108, new medium-wavelength waviness 109 and short-wavelength waviness 110 are formed on the surfaces of the starting material glass plate 101 through this rough polishing 104a, and hence it is necessary to increase the polishing amount in the pre-polishing 104b and the precision polishing 104c. In the conventional manufacturing method, there is thus a problem that it is necessary to initially make the starting material glass plate 101 thicker by a certain predetermined amount, and moreover a large amount of polishing waste is discharged during the polishing, resulting in an increase in the amount of industrial waste, and an increase in the manufacturing cost.
Furthermore, the abrasive grains used in the rough polishing 104a have a larger diameter than the abrasive grains used in the pre-polishing 104b and the precision polishing 104c, and hence the surfaces of the starting material glass plate 101 are easily scratched, and the amount of subsequent polishing must be increased to remove these surface scratches; there is thus a problem that the starting material glass plate 101 must initially be made thicker by a certain predetermined amount for this reason as well.
Moreover, because the rough polishing 104a using abrasive grains having a large diameter is carried out after the inner and outer peripheral surfaces of the starting material glass plate 101 have been ground and polished in the edge processing step 103, even if the inner and outer peripheral surfaces have been mirror-finished at considerable effort in the edge processing step 103, they are polished once again with coarse abrasive grains during the rough polishing 104a, and hence there is a problem that the surface roughness of the inner and outer peripheral surfaces drops, resulting in a drop in product quality.
Furthermore, because the surface polishing step 104 is carried out in three divided stages (the rough polishing 104a, the pre-polishing 104b and the precision polishing 104c) as described above, there is a problem that the number of steps required in the surface polishing is high and hence it takes a long time to complete the manufacturing, and moreover there is a risk of the surfaces of the starting material glass plates 101 being scratched due to the substrates contacting one another or the jig or the like in each of the steps, and thus productivity is poor.